The present invention relates to an apparatus for the thermal treatment of substrates, especially semiconductor wafers, with at least two adjacent heating elements that are disposed essentially parallel to one another and are each provided with a heating filament. The apparatus is in particular related to a rapid heating unit in which the substrates are subjected to rapid temperature changes.
In the semiconductor industry, it is known to thermally treat wafers during the process of manufacturing the same. For this purpose, generally so-called rapid heating units are utilized, such as are described, for example, in DE-A-19952017, which originates from the same applicant. These units include a reactor having lamps for heating the substrates (preferably only one substrate is disposed within the reactor), and generally, although not necessarily, a process chamber (preferably of quartz glass) that is transparent for the lamp radiation and that is disposed within the reactor and surrounds the substrate. The substrate is subjected via the lamp radiation within the reactor or the process chamber to a thermal treatment pursuant to a predefined temperature-time-curve in a defined process gas atmosphere or in a vacuum. For the process result of the thermal treatment, it is very important that the wafer be heated uniformly, and that a homogeneous temperature distribution result on the wafer surface, or that a predefined temperature distribution can be realized as well as possible. Deviations from a homogeneous temperature distribution over the substrate are especially advantageous for silicon wafers if the process temperatures exceed 1200° C. and the heating and cooling rates are greater than 50° C./s. Under these process conditions, it has been shown that in the region of the final temperature an approximately parabolic temperature distribution having a temperature difference of about 5 to 20° C. (as a function of the diameter of the wafer) over the wafer diameter provides the best process results with regard to freedom of slip. However, such uses with desired, defined non-homogeneous temperature distribution over the wafer or the substrate are more the exception, since these processes entail the greatest demands on the regulatability and the temperature measurement of the substrate temperature, which only the most modern plants today can fulfill.
Primarily during the heating and cooling phases, there occurs with disc-shaped wafers the problem of very non-homogeneous temperature distributions, especially in the edge region of the wafer, which cannot be controlled or can be controlled only inadequately. Thus, the edge of the wafer heats much more significantly and rapidly during the heating phase than does the inner portion of the wafer. This more rapid heating-up is due to the fact that at the edge of the wafer, a larger outer surface per volume of wafer is provided than in the interior of the wafer. Via this additional outer surface, the edge of the wafer absorbs more of the heat radiation than does the interior of the wafer (edge effect). Furthermore, the edge of the wafer is irradiated by a larger wall surface of the reactor, essentially via reflection of radiation, and “shadows” the interior of the wafer. Due to the reactor walls, the edge region of the wafer is thus irradiated that much more intensely the higher is the reflectivity of the wall surfaces. Thus, during heating-up of the wafer, the edge of the wafer, in addition to the pure “edge effect”, is additionally heated due to the presence of the reactor walls. Since the reactor walls of rapid heating units are usually cooled (cold wall reactors), and the wall temperature is generally less than 100° C., the reactor walls have a relatively low thermal inherent or characteristic radiation relative to the reflected radiation, as a result of which the influence thereof during usual process temperatures of greater than 400° C. can be disregarded.
On the other hand, during the cooling phases the wafer cools more rapidly at the edge of the wafer than in the interior of the wafer, since via the larger surface per wafer volume at the edge, more thermal radiation is emitted. In addition, surfaces of the reactor chamber that are disposed across from the substrate and are generally arranged parallel to the substrate reflect the radiation energy given off from the wafer back to the center of the wafer in a reinforced manner, thereby further slowing down the already slow cooling-off of the center of the wafer. This slowing down is that much greater the more reflective are the surfaces, or the more these surfaces radiate thermal energy. The influence of the edge of the wafer and of the process chamber walls upon the homogeneity of the temperature is also designated as the photon-box-effect, and is, among other things, essentially a result of the reflection of a portion of the heat radiation at the reflective chamber walls, and is included in the main problems during the rapid heating of semiconductor substrates, especially if during the entire duration of the process, in other words also during the dynamic phases of the heating-up and cooling-off, an as uniform as possible or a predefined temperature distribution (which itself can again be a function of the temperature) is to be achieved over the wafer.
From the aforementioned DE-A-19952017 it is known to surround the wafer with a compensation ring in order to reduce the photon-box-effect. In particular, the compensation ring is tilted as a function of the progress of the process in order to achieve a shadow effect relative to the lamps at the edge of the wafer. In addition to this approach, it is also known to provide light-transforming plates, also knows as hot-liners, parallel to the wafer in order to indirectly heat the wafer via such plates, and hence to reduce the photon-box-effect. However, these approaches can only partially reduce the photon-box-effect, and they lead to a complicated construction of the rapid heating unit.
In the known rapid heating units, rod-shaped tungsten-halogen heating lamps are generally utilized. The heating lamps are provided with a tungsten filament that is kept in a halogen-containing atmosphere. During the operation of the lamps, tungsten from the filament is volatilized and reacts with gas molecules to form tungsten halide. During the operation of the lamps below approximately 250° C., a condensation of the tungsten on the lamp tubes can occur, which, however, can be avoided if the lamp glass is kept in a temperature range between 250° C. and 1400° C. The condensation should be avoided, since a fog connected therewith on the glass adversely affects the heating process and the service life of the lamps. If the tungsten halide comes into the vicinity of the filament, sufficient thermal energy is applied to break the chemical bond and to again deposit the tungsten upon the filament. Subsequently, the halogen gas can repeat the process. This cycle is known as the halogen process.
With the conventional rod-shaped tungsten-halogen lamps, the filament extends approximately in the center of the lamp cross-section along the longitudinal axis of the lamp, and is uniformly spirally coiled essentially over the entire length of the lamp. Only in the end regions are linear filament sections provided for the transition into the respective lamp socket. As a result, an essentially uniform heating capacity can be achieved over the entire length of the lamp, which, however, contributes to the aforementioned photon-box-effect since, as mentioned above, with a uniform heating capacity over the surface of the wafer the edge region is heated more pronounced than is the central region.
With the aforementioned DE-A-19952017 the wafer that is to be treated is furthermore disposed in a process chamber that comprises quartz glass, whereby the heating lamps are disposed outside of the process chamber. The quartz glass is transparent for the radiation emitted from the heating lamps. After a heating of the wafer within the process chamber, the wafer emits a short wave thermal radiation in the range of 0.3 to 4 μm, as well as a longer wave thermal radiation in the infrared range of greater than 4 μm. The quartz glass of the process chamber is not entirely transparent for this longer wave thermal radiation of greater than 4 μm, and therefore a large portion of this thermal radiation is absorbed by the quartz glass. Thermal radiation that is not absorbed is reflected back to the chamber, and again a large portion is absorbed in the quartz glass. A remainder falls on the wafer and is absorbed thereby. Due to the absorption of the thermal radiation in the quartz glass, there is a localized heating-up of the process chamber, especially in a region of the process chamber that is disposed directly above or below the wafer. This effect is further reinforced by a reflection of the thermal radiation at the reflective chamber walls of the unit, since the thermal radiation is essentially reflected directly back to the wafer, so that a region of the process chamber that essentially corresponds to the projected shape (i.e. having the same circumferential shape) of the substrate is heated significantly more than regions disposed beyond this region. This process again reinforces the so-called photon-box-effect, especially if the process chamber is greatly heated up, so that it irradiates back to the wafer within the chamber. This return radiation prevents a rapid cooling of the wafer, especially in the middle of the wafer. The process chamber of quartz acts as a sort of energy trap for the long wave thermal radiation, whereby due to a coupling between wafer and process chamber the central region of the wafer is always irradiated more strongly, since the process chamber walls that are disposed approximately across from this region are at a higher temperature than are the other process chamber walls. This makes it clear that a non-homogeneous temperature distribution of the process chamber (e.g. of quartz) has an influence upon the temperature distribution of the wafer. For this reason, it is attempted to cool the process chamber as homogeneously as possible. However, the process chamber temperatures can readily reach a range of 600° C.
Proceeding from the aforementioned state of the art, the object of the present invention is to provide an apparatus for the thermal treatment of substrates, especially semiconductor wafers, that enables a more homogeneous or defined heating of the substrate that is to be treated.